Orthogonal Frequency Division Multiplexing (OFDM) is one widely utilized multi-carrier based scheme for wired or wireless wideband digital communication systems. One known implementation of OFDM is by the scheme of Cyclic Prefix OFDM (CP-OFDM) in which tail part of OFDM body is used to stuff the guard interval between two consecutive OFDM bodies. Since this CP (cyclic prefix) is actually part of OFDM body, both the CP and the OFDM-body are multi-carrier based. The frequency spectrums of CP and OFDM-body coincide with each other. In practice, this CP can be used to combat inter-symbol-interference (ISI) introduced by multi-path propagation of microwave and, sometimes, to assist timing/frequency synchronization. Additionally, dedicated pilot signals can/should be allocated on certain sub-carriers in this scheme for channel estimation to facilitate the coherent detection/demodulation as well as to assist timing/frequency/sampling synchronization too. Nevertheless, the foregoing advantages are also associated with tradeoff of spectrum efficiency because certain bandwidth is consumed by pilot signals which do not carry any user information.
One other known implementation of OFDM is by the scheme of Time Division Synchronous OFDM (TDS-OFDM) in which a single-carrier modulation based pseudo noise (PN) sequence, instead of tail part of the OFDM body, is stuffed in the guard interval. The pseudo noise (PN) prefix is single-carrier based while OFDM-body is multi-carrier based. Since the PN sequence enables timing/frequency/sampling synchronization as well as channel estimation, there is no dedicated pilot sub-carriers are needed for TDS-OFDM such that the spectrum efficiency is higher than CP-OFDM. The capability of TDS-OFDM has been well proven in broadcast environment, and TDS-OFDM is chosen as the key technology of Chinese Digital Terrestrial TV standard (DMB-T).
The application of TDS-OFDM beyond broadcasting is still under development. One of the challenges is to handle Frequency Division Multiple Access (FDMA), which is a common scheme in modern communication systems to allow two or more than two entities to share the sub-carriers of one OFDM symbol. The PN sequences from respective entities may interfere with each other.
Examples of OFDM applications in communication system include uplink Orthogonal Frequency Division Multiple Access (OFDMA), in which OFDM symbols are generated by different users that occupy different sub-carriers. OFDM can be applied to cellular system, different base stations can serve users on different sub-carriers to achieve frequency reuse factor larger than 1. OFDM also works well in multiple antennas system, in which different antennas may use different orthogonal sub-carriers to transmit pilot signals. While it is straightforward for CP-OFDM to implement multiple access schemes, however it is not implemented as well as for TDS-OFDM.
Chinese patent publication no. CN1893337 discloses an “Emission diversity method for time-domain orthogonal frequency dividing duplexing system” which uses orthogonal PN codes as the PN sequences of TDS-OFDM for different entities. As such, there is no interference between the entities even the PN sequences share the same frequency spectrum. This scheme was proposed to be used with continuous (adjacent) data sub-carrier allocation as well as discrete data sub-carrier allocation. However, the PN sequence is designed to be transmitted over all the sub-carriers. Power is wasted in the part of the spectrum that is not used to transmit the OFDM body of the corresponding entity.
Consequently, a need exists for improved methods and apparatus that can implement FDMA on OFDM and provide full utilization of the transmission spectrum and power.